As the largest organ in the human body, the human skin performs many vital functions. With an average surface area of approximately 2 m2 in adults, it has a prominent role as a protective and sensory organ. It is the job of this organ to communicate and repel mechanical, thermal, actinic, chemical and biological stimuli. It also has an important role as a regulatory and target organ in the human metabolism.
Cosmetic skin care is primarily concerned with reinforcing or restoring the natural function of the skin as a barrier against environmental influences (e.g. dirt, chemicals, microorganisms) and against the loss of bodily substances (e.g. water, natural fats, electrolytes) and to support the natural regeneration ability of its stratum corneum if it becomes damaged. If the barrier properties of the skin are disrupted, this can lead to greater resorption of toxic or allergenic substances or to attack by microorganisms, resulting in toxic or allergenic skin reactions. A further objective of skin care is to compensate for the loss of fat and water in the skin caused by daily washing. This is especially important if the natural regeneration ability is not adequate. Skin care products should also protect against environmental influences, particularly sun and wind, and delay skin ageing.
Medical topical compositions generally contain one or more drug products in an active concentration. In the interests of simplicity, in order to obtain a clear distinction between cosmetic and medical usage and corresponding products, reference is made to the legal provisions of the Federal Republic of Germany (e.g. the German Cosmetics Ordinance, Food Act and Drugs Act).
Emulsions are understood to be heterogeneous systems comprising two liquids which are immiscible or have only limited miscibility with each other and which are conventionally known as phases. In an emulsion, one of the two liquids is dispersed in the other liquid in the form of extremely fine droplets. If the two liquids are water and oil and oil droplets are finely dispersed in water, it is an oil-in-water emulsion (O/W emulsion, e.g. milk). The basic character of an O/W emulsion is shaped by the water. In a water-in-oil emulsion (W/O emulsion, e.g. butter), the principle is reversed and the basic character is determined by the oil. The person skilled in the art is naturally aware of many possibilities for formulating stable O/W preparations for cosmetic or dermatological use, for example in the form of creams and ointments which are spreadable in the range from room temperature to skin temperature, or as lotions and milks, which are more flowable in this temperature range. These systems generally contain emulsifiers.
Emulsifiers are auxiliary substances for the preparation and stabilisation of emulsions which are generally in the form of oily to waxy, but also powdered, substances. In order to stabilise emulsions over an extended period, emulsifiers are needed which suppress or delay the separation of the two phases, e.g. oil and water, to the thermodynamically stable final state until the emulsion has fulfilled its purpose.
Emulsifiers lower the interfacial tension between the two phases and as well as reducing interfacial activity also stabilise the emulsion that is formed. Emulsifiers stabilise the emulsion that is formed by means of interfacial films and by forming steric or electrical barriers, preventing the flowing together (coalescence) of the emulsified particles. Both the elasticity and the viscosity of the interfacial films are important factors in emulsion stabilisation and are strongly influenced by the emulsifier. Stabilising an emulsion which has already been formed is the most important property of emulsifiers, and more important than facilitating the primary distribution of the phases, since an adequate number of mechanical aids are available for this purpose. The most important requirements of emulsifiers are as follows:                a) The emulsifier must concentrate at the boundary layer between the phases. To this end it must have interface-active or surface-active properties, in other words reduce the interfacial tension of the immiscible phases.        b) The emulsifier must also either charge the particles so that they repel one another or form a stable, often highly viscous or even solid protective layer around the particles.        
These properties are sufficient in themselves for many applications. For the preparation of emulsions which are stable over a particularly long period, however, the creaming or sedimentation of the dispersed particles must be prevented and their tendency to coalesce still further reduced. This is achieved by increasing the viscosity of the outer phase and/or forming protective viscous structures, e.g. liquid crystalline or gel phases. In this case, in addition to the emulsifier itself, the emulsifier system must also contain an additional component known as a co-emulsifier, stabiliser or also, depending on the active mechanism, as a consistency modifier or protective colloid.
In order for compounds to be effective as an emulsifier, they must exhibit a particular molecular structure. The structural feature of such compounds is their amphiphilic molecular structure. The molecule in such a compound has at least one group with affinity to high-polarity substances (polar group) and at least one group with affinity to non-polar substances (apolar group). The polar group is a functional group whose electron distribution gives the molecule a considerable dipole moment. This group determines the affinity to polar liquids, particularly the affinity to water, and the hydrophilic nature of the compound. For that reason the polar function is also called the hydrophilic group.
The apolar group, on the other hand, is the part of the molecule whose electron distribution makes no significant contribution to the dipole moment. The apolar group determines the affinity to apolar liquids, particularly low-polarity organic solvents, for which reason this function is also called the lipophilic group. The combined presence of hydrophilic and lipophilic groups in the molecule allows emulsifiers to interact with both hydrophilic and lipophilic phases. This leads to an orientation at the interface which is the precondition for the interfacial activity of such compounds.
A decisive feature for the characterisation of emulsifiers is the ratio of hydrophilic to lipophilic constituents in the molecule, expressed as the hydrophilic-lipophilic balance (HLB system), for the determination of which there are many experimental and theoretical mathematical methods, all of them based on determining the ratio of hydrophilic to lipophilic groups in the molecule component. Emulsifiers with a high HLB value produce O/W emulsions, those with a low HLB value preferentially form W/O emulsions.
However, only a few O/W emulsifiers are known from the prior art which form such low-viscosity O/W emulsions that these would be sprayable, for example. Most of them contain polyethylene glycols. Furthermore, low-viscosity O/W emulsions of the prior art commonly have the disadvantage of being unstable, limited to a narrow range of applications or to a restricted choice of constituents. For that reason there are currently no low-viscosity, cold-processable O/W emulsions on the market in which for example highly polar oils—such as the plant oils otherwise commonly used in commercial products—are adequately stabilised. According to the prior art, O/W emulsions having a low viscosity and displaying a storage stability such as is required for marketable products can be formulated only with great difficulty. The choice of such formulations is accordingly extremely limited. Nevertheless, such formulations could offer the consumer previously unknown cosmetic benefits.
A low-viscosity cosmetic or dermatological O/W emulsion is known from EP 1 049 452 which contains one or more partially neutralised esters of monoglycerides and/or diglycerides of saturated fatty acids with citric acid and one or more fatty alcohols chosen from the group comprising branched and unbranched alkyl alcohols having 12 to 40 carbon atoms.
US 2003/0012801 discloses a special emulsion which contains one or more partially neutralised esters of monoglycerides and/or diglycerides of saturated fatty acids with citric acid, cyclodextrins and retinoids.
A hair care agent is known from WO 99/62468 which in addition to a special pentaerythritol ester oil contains a second oil component. Citric acid ester oils having a very generalised formula with unsaturated alkyl radicals having between 1 and 30 carbon atoms are claimed inter alia as an example for the second oil component. Specific compounds from this class of substances are not listed.